(Applicant's Abstract) The susceptibility to allergic inflammatory diseases, including asthma, is determined by a complex set of genes that contribute to the regulation of airway function and inflammation. Agents that increase intracellular cAMP have predominantly inhibitory effects on the signaling pathways that control pro-inflammatory cell and airway smooth muscle function. An increase in cAMP is associated with the inhibition of airway smooth muscle contraction, T cell activation, and the migration and/or function of the effector cells that are present in sites of allergic inflammation, such as mast cells, eosinophils, and basophils. Here, we will test the hypothesis that genes coding for cyclic AMP-specific phosphodiesterases (PDE4s), components of the cAMP signaling cascade, are major determinants of allergen-induced airway hyperreactivity (AHR). Of the four PDE4 genes present in humans and other mammals (PDE4A, B, C, and D), three (A, B, and D) are expressed in the airway and in inflammatory cells. We have established in vivo models in which these PDE4 genes have been inactivated by homologous recombination, and have shown that allergen-induced AHR does not develop in mice homozygous null for either of two of these genes (PDE4B or PDE4D). These findings indicate that these PDEs, and the signaling functions they serve, play an essential role in the pathogenesis of AHR, a hallmark of asthma. However, while both PDE4B -/- and PDE4D -/- mice exhibit loss of allergen-induced AHR, only PDE4D -/- mice exhibit no airway responsiveness to cholinergic stimulation at baseline. This finding strongly suggests that the cellular basis of the AHR phenotypes in the two mutant mice may be distinct. We now propose to use these in vivo mouse models to define in detail how PDE regulation and cAMP homeostasis can influence the biology of the airway smooth muscle and pro-inflammatory cells. Taking advantage of these in vivo models, we will investigate how PDE4 isoenzymes and, more generally, cAMP signaling, can influence, a) the differentiation and recruitment of inflammatory cells to the lungs, and b) the development of specific characteristics of asthma, including AHR. These experiments in mouse models of asthma will permit us to test the concept that polymorphisms/mutations in the PDE4 alleles present in the human population contribute to the genetic background predisposing to allergic asthma. They will also provide "proof of concept" for the development of a new class of drugs useful for the treatment of asthma and other inflammatory disorders.